1. Field of the Invention
The present invention relates to a fluoride copolymer, a polymer electrolyte and a lithium battery employing the polymer electrolyte. More particularly, the invention relates to a fluoride polymer, to a polymer electrolyte that can be used for electrochemical devices such as lithium batteries and that has good mechanical properties and ionic conductivity characteristics, and to a lithium battery having improved charging/discharging characteristics.
2. Description of the Related Art
According to the rapid development of various electronic devices and computer-related industry, there has been an increasing demand for highly efficient secondary batteries. To meet the demand, characteristic requirements of secondary batteries include, for example, safety, portability, compactness, and the like.
Typical batteries satisfying the foregoing requirements are lithium polymer batteries that most expect to be the next-generation secondary batteries. Lithium polymer batteries can overcome several shortcomings of lithium ion batteries that currently are being put into widespread use, including such shortcomings as safety problems, high material costs, difficulty in manufacturing large scale batteries, and difficulties in attaining high-capacity batteries. Development of polymer electrolytes with high ionic conductivity, good mechanical properties for better manageability, and high electrochemical stability is a prerequisite for achieving practicality of lithium polymer batteries.
Recently known polymer batteries include hybrid polymer electrolytes containing an organic electrolytic solution, gel-type polymer electrolytes and the like. Hybrid polymer electrolytes can minimize problems encountered in liquid electrolytes. However, they still have several disadvantages including a complicated production process due to the need to extract a plasticizer or impregnate an electrolytic solution, a safety problem in view of battery quality such as leakage, a difficulty of ensuring reproducibility, and the like.
Gel-type polymer electrolytes, on the other hand, can be prepared by mixing solid polymers and organic electrolytic solution at an initial manufacture stage to be cast. Thus, there is no need to perform a subsequent injection process of an electrolytic solution. Also, since the electrolytic solution is mixed with solvents from the initial manufacture stage, the electrolytic solutions can be uniformly distributed throughout the battery. Further, since a large amount of liquid electrolytes are impregnated into a polymer matrix after casting, the ionic conductivity can advantageously be increased.
Examples of the gel-type polymer electrolytes include polymer electrolytes prepared by adding organic electrolytic solutions to polymer resins such as polymethylmethacrylate (PMMA), polyacrylonitrile (PAN) or polyvinylchloride (PVC). Such a polymer electrolyte is comprised of a polymer resin having a bipolar moment, in which lithium salts are dissolved in an organic solvent, and exhibits high ionic conductivity of 10−3 S/cm or higher, at room temperature, when the organic solvent and the lithium salts are mixed at an optimum ratio. However, gel-type polymer electrolytes still have several problems, such as a complicated battery manufacturing process, or an increased cost because the manufacturing process requires a high temperature drying step at 100° C. or higher, and the resin has a high viscosity when it is molten.
Another example of gel-type polymer electrolytes is disclosed in Japanese Patent JP03-207752A, in which a polymer electrolyte is prepared by mixing ethylene glycol and dimethacrylate and subjecting to the mixture UV radiation. The gel-type polymer electrolyte has an ionic conductivity of 10−4 S/cm or less, is too soft, and is thermally cured upon UV radiation, disabling further formation. Also, during battery fabrication, a gap forms between each electrode and the polymer electrolyte, and it becomes relatively larger thereby increasing the interface resistance between the electrode and polymer electrolyte, making the gel-type polymer electrolyte difficult to be practically used for secondary batteries.
U.S. Pat. No. 4,830,939, the disclosure of which is incorporated by reference herein in its entirety, discloses a polymer electrolyte prepared by mixing a polymerizable monomer having one or more unsaturated functional groups and an electrolytic solution and curing the same by UV radiation. Although this polymer electrolyte has good ionic conductivity, it undesirably has poor flexibility.
U.S. Pat. No. 5,463,179, the disclosure of which is incorporated by reference herein in its entirety, describes a method of improving the ionic conductivity of a polymer electrolyte by providing a three-dimensional stable space between molecules of a polymer by introducing relatively rigid functional groups into a polymer matrix forming the polymer electrolyte. However, according to this method, even though the ionic conductivity of the polymer electrolyte is increased to approximately 4×10−3 S/cm, the interface resistance between the electrode and polymer electrolyte is relatively increased to cause a deterioration in battery performance, thereby making the polymer electrolyte difficult to be practically used.
The description herein of disadvantages of various known systems, methods, and apparatus is not intended to limit the scope of the present invention. Indeed, certain aspects of the present invention may include one or more features from known methods, apparatus, and systems without suffering from the same disadvantages.